The photographic material for photomechanical process is used for making dot images or line images which is necessary to make a printing plate using photomechanical process.
In many cases such a photographic material for a photomechanical process is submitted to a particular type of processing called reduction processing at the final stage of forming a dot or line image in order to finely retouch part or all of the image for the purpose of reproducing a subtle tone or satisfying artistic presentation.
Therefore, it is very important for the photographic material for a photomechanical process to have a reduction processing aptitude.
An example of a method for carrying out reduction-processing of a photographic material for a photomechanical process which has dot images or line images formed through optical exposure and development-processing involves bleaching the metal silver, which constitutes the dot or line images, with an oxidizing agent.
Specifically, C. E. K. Mees, The Theory of the Photographic Process, pp. 738-739, Macmillan, New York (1954) describes reducers which use reducing components such as a permanganate, a ferric salt, a ceric salt, a hexacyanoferrate (III), a dichromate, a persulfate and so on.
Reduction processing is, in the ultimate analysis, oxidization of the silver image and therethrough, solubilization of the silver image. Therefore, the extent to which the image can be retouched by reduction processing generally increases with the quantity per unit area of silver which constitutes the image.
More specifically, when reduction processing a dot image, diminishing of the dot area due to the reduction processing is, in general, accompanied by a decrease in the blackening density of the dots. Accordingly, the smaller the decrease in blackening density to a dot upon diminution of the dot area is, the greater the extent to which the image can be retouched by reduction.
In other words, the extent to which a dot image can be retouched is represented by the extent it is possible to diminish the dot area blackening density of a dot while maintaining the density at some definite value or higher.
In the present specification, the extent of diminution of the dot area when its blackening density is decreased by a reduction processing to the lowest value which can be used in the photomechanical process, compared with the dot area prior to the reduction processing, is expressed in terms of the reduction width.
As described above, the larger the amount of silver which constitutes image is, the wider the reduction width becomes. Consequently, the greater the retouchable extent of the image due to the reduction processing becomes.
At first, it would appear as though a good solution to the problem would be to increase the amount of silver halide to be coated per unit area of the photographic material for a photomechanical process. However, silver is very expensive and valuable and consequently, it is undesirable to increase the coverage of silver.
Because of these circumstances, one of important problems in the art has been to contrive methods for imparting all of the necessary characteristics for a photographic material for a photomechanical process while using silver in the smallest possible amount.
In general, a photographic material for a photomechanical process has a gelatin protective layer on a silver halide emulsion layer. However, an amount of a hydrophilic colloid contained in the gelatin protective layer is, in general, 0.5 or less times that of a hydrophilic colloid contained in the silver halide emulsion layer. For example, U.S. Pat. No. 3,600,174 discloses a high-contrast photographic material wherein 130 mg/ft.sup.2 or more of gelatin is coated on a silver halide emulsion layer. However, the maximum amount of gelatin coated on the silver halide emulsion layer in said U.S. Patent is only a slightly higher than 0.5 times the amount of gelatin contained in the silver halide emulsion layer. Such a maximum coating amount of hydrophilic colloid of the gelatin protective layer cannot improve reduction processing aptitude of the photographic material.